FIX: Include auth_providers for anonymous users when login_required

app/models/site.rb

@@ -91,6 +91,9 @@ class Site
         filters: Discourse.filters.map(&:to_s),
         user_fields: UserField.all.map do |userfield|
           UserFieldSerializer.new(userfield, root: false, scope: guardian)
+        end,
+        auth_providers: Discourse.enabled_auth_providers.map do |provider|
+          AuthProviderSerializer.new(provider, root: false, scope: guardian)
         end
       }.to_json
     end